Embryonic chick duodenum, maintained in organ culture in a defined medium responds to vitamin D-like steroids in vitro similarly to the intact chick intestine: cAMP elevation, increased RNA synthesis followed by de novo induction of a specific calcium binding protein (CaBP) and, concomitantly, increased transmucosal calcium transport, followed by elevated alkaline phosphatase activity and, finally, enhanced phosphate transport. CaBP biosynthesis in response to the potent metabolite, 1 alpha, 25-(OH)2-D3, is undoubtedly de novo and regulated in a complex manner by cAMP and Ca2 ions. While PTH and CT have no effect on vitamin D-mediated responses, the hormones hydrocortisone (HC) and triiodothyronine (T3) augment CaBP induction. Experiments using this system have established an important, if not singular, role of CaBP in calcium transport in the intestine chiefly by stimulation of diffusional calcium influx at the mucosal surface. Recent work has demonstrated the induction of metallothionein (MT) by either Cd2 ion or Zn2 ion added to the culture medium even while 1 alpha,25-(OH)2-D3-mediated responses were inhibited. Work in progress includes development of a culture medium permitting "normal" cell proliferation by addition of known or suspected proliferogenic agents (mainly hormones). Work will continue on the synergistic action of HC, T3 and Zn2 ion on 1 alpha,25-(OH)2-D3-mediated responses, on the role of cAMP in CaBP biosynthesis, on the action of a variety of agents known or suspected to alter calcium absorption in vivo, and on the use of the culture technique in vitamin D and anti-vitamin D analog assay. The obvious utility of the system in assessing the effects of toxic agents (e.g., Cd2 ion and other metals) will be exploited further including isolation and characterization of induced MT(s). The new filter paper method for measuring mucosal surface uptake will be utilized to study unidirectional fluxes of calcium in conjunction with the organ culture technique. Finally, efforts will be instituted to isolate and purify CaBP-mRNA for use in reconstitution experiments and, perhaps, to develop a cDNA probe for CaBP-mRNA analysis.